The length of time you're carrying debt is a factor to consider. 

Other factors such as the quality of the tenant, and location will also determine the value you can get.

@Isadore Nelson Yes, eviction is possible, but there are too many factors that need to be taken into consideration, active military family members, disabled veterans, building violations, maintenance issues and etc...

I factor in 5% vacancy, 5 % cap ex, and maybe $1600 lawncare and maintenance for all 4 properties.  

Run your comps and be prepared to adjust a suggested price based on condition, layout, and other factors.

I think pools also factor in to summer rentals, if you don't have one that will hurt you. 

Of course, if you do flip it you'll want to put in nicer finishes so that would increase your rehab costs and that needs to be factored in as well. 

The load calculation made for the existing foundation were not factored into the initial design.

I base the price on a few factors - Room size, amenities and area average rents. 

So most investors just throw in the same expense factors no matter the age of the property.